Electrical damping device for electromechanical transducers



April 1o, 1962 P. RENAUT 3,029,356

ELECTRICAL DAMPING DEVICE FOR ELECTRO-MECHANICAL TRANSDUCERS Filed Oct. 30. 1956 Paul Renan/'IL United States Patent G This invention relates to electrical damping devices for `electro-rnechamic'al' transducers: ymanyy applications it may be "necessary todar'rip electro-mechanical transducers, in orderihat theyouickly 'stop oscillating after Vthe signal causedthem to oscillate has been cutol. i i` The'manner in which this result is achieved in accordance withIt-he known varllt leaves much to be desired.v vFor example, piezoeleetric quartz crystal may be damped by lining ,the fa s I"ereofvitli',a` material having a high mechanical'"impedaceIsuch as" raldite. l However, a mechanical damping isgeixerallyy not satisfactory.

It has also been lsugges-t`e l`to use electric damping by transmittihg'to the/.vibrating ihsember'ablocking pulse 'offlco'nvenint amplitude', signand phase with respect to the'eiiciterV pulsejsolasto block the mechanical oscillation caused b y the rlatter.` l

A'This process is hijdlyhapplicable in an eiiicient Way, v

since the blocking pulse parameters should be adjusted in accordance jiviththe'coiitaet impedance between the os llating member and the medium' to which the me,- chical'vib'rations yaretransrn'itted; now, this impedance may be,V quite'vafriable and it would lnot be practicable is 'adjust mangaring 'pulses as 'this 'impedaace'varies The 4inyer'itionproj'zides a noyel' system for electrically damping a`n electro-mechanical transducer.

Theinvention willbe best understood' from the following descriptionv with reference tov thev appended drawing, theonly FIGURE of whichshows by way of example a"systein-accordingt` the invention.u i Y In the drawing,`1 is ar souree of electrical energy," 2 an electrofmechanical transducer device suitable for ytransformingelectricl energy into mechanical energy, 3 is a transformer whose secondary winding is coupled to the output of an amplifier' 4, S a, ontrol device for controlling mplier 4'; 6 and? are adjustable condenser and resistance'respectively. Y v As shown in theiigure, the input of amplifier 4 is coupled to the midpoint E of the primary Winding` CD of 'transformerv 3. 4 i W l l, v By way of example, it will be assumed that transducer 2 is a quartz'cr'ys'tal. It is connected Vacross terminals A andl B andpas itis known, when oscillating at its natural.

frequency, is' electrically equivalent to a resistance S in paralisi with a condenser 9 and'ifn series with an electric source 10 having af nil internal resistance and generating a constant current.v Thewcir'cuit l1 according to the invention is4 connected across terminals A and B. The impedance 'across' terminals 'Ar and B as seen from source 1l), is the impedance L Go of condenser 9 This impedance will be hereinbelow calculated in the. actual. Case which has been illustrated in the drawing. v

Il and l2 being the currents,`taken positive in the direction of-'the arrow, inr resistance 8 andV in the primary coil CD ofy transformer 3f respectively, it can .be written:

rice

2 where VA, VC, VD are the potentials at points A, lC and D respectively, C the capacitance of condenser 9, and Z the impedance consisting of condenser 9 and resistance S (or condenser 6 and resistance '7 identical to the latter respectively). However in practice resistance 7 may be disregarded with respect to capacitance and, accordingly may be considered as substantially equal to Z.

Accordingly one can write:

Thus potential VE at the mid point E of winding CD will be:

lt should be noted that current Ig'is the current induced in winding CD by the winding of transformer 3, coupled to the output of amplifier 4, provided the current .fedto Winding CD by source 10' may be disregarded. This approximation is quite justifiable if the impedance of amplifier 4, as seen from source l@ across terminals CD, is high. This will be the case for example if the amplification chain 4 includes voltage amplifier stages having an amplification coeicient A and followed by a power amplifier stage with a pentode valve 13 having a transconductance p and equivalent to a source of constant current equal to p A/E.

Under these conditions, the constant current source 4 has a very high output impedance and accordingly estabiishes a very high impedance across terminals CD. Therefore, the current ilowing through coil CD from source 10 is practically nill, and one can Write:

' Thus, due to the provision of circuit ll, the impedance seen from source 210 across terminals A and B is multiplied by factor Vcant does not rely on the( theoretical explanation which has been given above. In particular, it is not necessary for the operation of the device that amplier 4 should include a constant current output stage.

It should' be noted that the arrangement of the gure may be viewed as comprising a bridge circuit with two arms in parallel, namely ECAB to ground and ED to ground a feedback loop. The two arms of the bridge are balanced, one including the transducer and the other the electrical equivalent thereof and the feedback loop comprises ampliter 4. Thus the output voltage of the loop which is reinjected to the bridge (and makes the impedance across terminals CD as seen from transducer 2 very high to prevent the transducer from oscillating), is not in turn partially re-injected into the feed-back loop.

In other words, according to the invention, a feedback loop inserts a very high impedance across the electromechanical transducer terminals, the loop being connected across these terminals through a balanced bridge circuit which prevents the loop from going into oscillation.

For example, in the case illustrated in the drawing, it has been shown that Therefore, the voltage applied to amplifier 4 input does not depend upon I2 and any self-oscillation is avoided.

It has been priorly taught to use an inductance coil forming with condenser 9 a tank circuit tuned at the working frequency of the crystal. This simple solution has an obvious drawback: the time constant of such a tank circuit is high and, therefore, its damping efi`ect will become effective only after a given number of oscillations of the transducer. Besides the tank circuit will go on oscillating for some time after damping of the transducer. Thus this system would be practically difficult to use.

As a practical application ofthe device shown in the tigure of similar devices, one may mention the damping of ultrasonic pulses transmitted by a piezofelectric transducer i to a body to be examined. lt is known that this dam-ping is quite a problem and that each pulse transmitted is generally followed by spurious pulses of decreasing amplitude which render difficult the vexamination of the echoes on the receiver cathode ray tube screen.

According to one aspect of the invention, the electric pulse generator 1 which energizes quartz crystal 2 is also used to make conductive the normally blocked amplifier 4. This may be done through delay line which is, for example, connected `to the control grid 12 of the above mentioned pentode 13. By adjusting the delay caused by this line, amplifier 4 may be made conductive after a time equal, for example, to the half oscillation period of the crystal. The quartz crystal then produces ultrasonic pulses free of any spurious oscillation.

As further application examples, an arrangement according to the invention may be applied to the damping of a loud-speaker or microphone membrane, of a piezoelectric ceramic, etc. These examples are of course not limitative.

What is claimed is:

1. A system for damping the vibration of an electromechanical transducer, said system comprising: a first circuit comprising a rst arm into which said transducer is coupled and a second arm having the same impedance as said transducer; a further circuit coupled to said first circuit and comprising a constant current source; and means for selectively switching said source in and out.

2. A system for damping the vibration of an electromechanical transducer, said system comprising: a first circuit comprising a first arm into which said transducer is coupled and a second arm having the same impedance as said transducer; a further circuit coupled to said first circuit; said further circuit comprising an amplifier; means for feeding energy to said amplifier; and control means for selectively blocking said amplifier and making the same conductive. v

3. A system for damping the vibration of an electromechanical transducer, said system comprising: a first circuit comprising a first arm into which said transducer is coupled and a second arm having the same impedance as said transducer; a transformer having a primary cir cuit having a midpoint and a secondary circuit, said primary circuit being coupled in Series between said two arms; an amplifier having an input and an output coupled to said secondary circuit; a feedback between said midpoint and said input; and means for selectively blocking and making conductive said amplifier.

4. A system for damping the vibration of an electromechanical transducer, said system comprising: a first circuit comprising a first arm into which said transducer is coupled and a second arm having the same impedance as said transducer; a transformer havingV a primary circuit having a midpoint and a secondary circuit, said primary circuit being coupled in series between said two arms; an amplifier having an input and including an output stage, having a tube having a control electrode and an output, the output impedance of said stage being high; the output of said stage being coupled to said secondary circuit; a feedback loop between said midpoint and said input; and means for biasing said electrode for selectively blocking said tube and making the same conductive.

5L A system for damping the yvibrations of an electro mechanical transducer controlled by a pulse generator, said system comprising: a first circuit comprising a first arm into which said transducer is coupled and a second arm having the same impedance as said transducer; a further circuit coupled to said first circuit and comprising a constant current source; and means for selectively switching said source in and out, said means being controlled by said pulse generator and including delay means for selectively delaying the controlling action of the pulses generated by said generator. g

6. A system for damping the vibrations of an .electromechanical transducer Vcontrolled by a pulse generator,

said system comprising: a firstcircuitcomprising a first arm into which said transducer is coupled and a secondarm having the same impedance as Vsaid, transducer; a transformer having a primary circuit having a midpoint and Ya secondary circuit, said primary `circuit being coupled in series between said two arms; an amplifier having an input and an output coupled tosaid secondary circuit; a feedback loopbetween said midpoint and said input; and means for selectivelyblockingand making conductive said amplifier; said means being controlled by said pulse generator and .including delayV means for selectively delaying the controlling action of the pulses generated by said generator.

7. A system for damping the vibrations of an electromechanical transducer controlled by a pulse generator, said system comprising a first circuit comprising@ first arm into which said transducer is coupled and a second arm having the same impedance as said transducer; `a transformer having a primary circuit having a midpoint and a secondary circuit, said primary circuit being coupled in series between said two arms; an amplifier having an input, and including an output stage, having a tube having a central electrode, and an output, the output impedance of said stage being high; the output of said stage being coupled to said secondary circuit; a feedback between said midpoint and said input; and means for biasing said electrode for selectively blocking said tube and making the same conductive; said means being controlled by said pulse generator and including delay means for selectively delaying the controlling action of the pulses generated by said generator.

References Cited in the le of this patent Y UNITED STATES PATENTS 

